Bhagavad Gita 18.6
But they should be done with detachment and without thought of reward. This is my final judgement.
एतान्यपि तु कर्माणि सङ्गं त्यक्त्वा फलानि च।कर्तव्यानीति मे पार्थ निश्िचतं मतमुत्तमम्॥etānyapi tu karmāṇi saṅgaṃ tyaktvā phalāni ca|kartavyānīti me pārtha niśicataṃ matamuttamam||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).